1. Field of the Invention
The present invention relates to iodonium salts of reduced crystallization tendency containing urethane groups, to a process for their preparation, and to their use for the radiation curing of cationically curing compositions.
2. Description of the Related Art
Cationic photopolymerization is a rapid, efficient and environment-friendly means of curing cationically polymerizable monomers. Particularly efficient photoinitiators are diaryliodonium salts (I) and triarylsulfonium salts (II).
Arxe2x80x94I+xe2x80x94ArMXnxe2x88x92xe2x80x83xe2x80x83(I)
xe2x80x83MXnxe2x88x92=BF4xe2x88x92,PF6xe2x88x92,AsF6xe2x88x92,SbF6xe2x88x92
Diaryliodonium salts (I) in particular are known from the patent literature (DE-A-25 18 639, U.S. Pat. No. 4,279,717, EP-A-0 334 056, EP-B-0 618 919) and are used as photoinitiators for polymerizing cationically polymerizable substances. The cationically polymerizable substances, however, have little or no polarity, especially if the polymerizable groups are present in organopolysiloxanes. When adding these photoinitiators, therefore, it is a very common observation that, depending on the structure of the formulation, the miscibility and solubility of the photoinitiators is limited. For this reason, the aryl radicals of such onium salts are often substituted with alkyl chains in order to increase the solubility in organopolysiloxanes (U.S. Pat. No. 4,310,469 and U.S. Pat. No. 4,374,066).
In the case of hydroxyl-bearing iodonium salts as described in U.S. Pat. No. 5,073,643, the poor solubility in nonpolar media is attributed to the high crystallization tendency. The particular complexation characteristics of the hydroxyl-bearing iodonium salts of the general formula (III) result in a strong crystallization tendency of the compounds (A. Kunze, U. Mxc3xcller, K. Tittes, J. P. Fouassier, F. Morlet-Savary, J. Photochemistry and Photobiology A: Chemistry, 110, 115-122 (1997)): 
The two oxygen atoms in the molecule act as ligands for a second iodonium ion. This aggregation behavior promotes the formation of crystals.
In the preparation of these salts, the strong crystallization tendency is entirely desirable, since it means that the compounds can be recovered as powders in a high purity by simple recrystallization. In this way, they are easy and cost effective to prepare. Such iodonium salts are commercially available, for example, under the name CD-1012 from Sartomer.
The high crystallization tendency has an adverse effect, however, if the iodonium salts are to be dissolved in nonpolar media, such as organopolysiloxanes. In this case either they are insoluble or a solid precipitate forms after just a short time.
Because of their inhomogeneity, such coatings cure poorly on UV irradiation. However, it is also possible that, as a result of the inhomogeneity, massive surface defects (craters, wrinkles, specks, etc.) appear even during the application of a thin coat on a substrate.
It is an object of the present invention to modify hydroxyl-bearing iodonium salts in a particularly cost-effective and simple manner such that the crystallization tendency is greatly reduced and good compatibility, especially with organopolysiloxanes containing epoxy groups, is established, and the resulting compounds are stable to hydrolysis, so that reformation of hydroxyl-bearing iodonium salts in the course of storage is impossible.
These objects are achieved by means of iodonium salts of the general formula (IV)
[R1xe2x80x94Ixe2x80x94R2]+Xxe2x88x92xe2x80x83xe2x80x83(IV)
where
I is iodine,
Xxe2x88x92 is an anion of a complex metal salt and/or of a strong acid,
R1 is the radical 
xe2x80x83in which
Ar is an aromatic hydrocarbon radical, preferably, having 6 to 14 carbon atoms per radical or is an aromatic hydrocarbon radical containing at least one oxygen and/or sulfur atom and, preferably, having 5 to 15 ring atoms per radical,
a is 1, 2 or 3,
b is 0, 1 or 2,
c is 0, 1 or 2,
D, E and F are each substituents of Ar,
D being a radical of the formula 
xe2x80x83where
x is 0 or 1,
y is 0 or 1,
R3 is a linear or branched divalent hydrocarbon radical, preferably, having 1 to 40 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom and/or at least one sulfur atom and/or at least one carboxyl group, R4 is a monovalent linear, branched and/or cyclic alkyl, aryl, haloalkyl and/or haloaryl radical, preferably, having 1 to 40 carbon atoms, which can be interrupted, if desired, by at least one oxygen atom and/or at least one sulfur atom and/or at least one urethane group and/or ester group, and which can, if desired, contain hydrophobicizing substituents and/or at least one isocyanate group,
E is a radical of the formula
xe2x80x94Oxe2x80x94R5
F is a radical of the formula
xe2x80x94R6
R2 is a radical of the formula 
xe2x80x83where
R5 is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom,
R6 is a monovalent hydrocarbon radical, preferably, having 1 to 18 carbon atoms per radical, which can be interrupted, if desired, by at least one oxygen atom,
d is 0, 1 or 2, and
e is 0, 1 or 2.
Surprisingly it has been found that the formation of a urethane from hydroxyl-containing iodonium salts makes it possible to reduce considerably the crystallization tendency, to improve substantially the compatibility with organopolysiloxanes containing epoxy groups, and to increase considerably the hydrolytic stability relative to iodonium salts containing Sixe2x80x94Oxe2x80x94C bonds (see German Patent Application 19901531.7).
Preferred examples of aromatic hydrocarbon radicals Ar are the phenyl, naphthyl and anthryl radicals.
Preferred examples of aromatic hydrocarbon radicals Ar containing at least one oxygen and/or sulfur atom are the 2-furyl, 3-furyl, 2-thienyl and 3-thienyl radicals.
Preferred examples of the divalent hydrocarbon radical R3 which can be interrupted by at least one oxygen atom and/or at least one sulfur atom and/or a carboxyl group are xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH(CH3)xe2x80x94, xe2x80x94CH2xe2x80x94CH(CH2CH3)xe2x80x94, xe2x80x94CH2xe2x80x94CH((CH2)3CH3)xe2x80x94, xe2x80x94CH2xe2x80x94CH((CH2)11CH3)xe2x80x94, xe2x80x94CH2xe2x80x94CH((CH2)13CH3)xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94Sxe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94C(O)xe2x80x94CH2xe2x80x94.
Preferred examples of R4, the monovalent linear, branched and/or cyclic alkyl, aryl, haloalkyl and/or haloaryl radical having 1 to 40 carbon atoms, which can be interrupted, if desired, by at least one oxygen atom and/or at least one sulfur atom and/or at least one urethane group and/or ester group, and which can, if desired, contain hydrophobicizing substituents and/or at least one isocyanate group, are alkyl radicals, such as the methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl, and tert-pentyl radical; hexyl radicals, such as the n-hexyl radical, for example, the cyclohexyl radical; heptyl radicals, such as the n-heptyl radical; octyl radicals, such as the n-octyl radical and isooctyl radicals, such as the 2,2,4-trimethylpentyl radical; nonyl radicals, such as the n-nonyl radical; decyl radicals, such as the n-decyl radical; dodecyl radicals, such as the n-dodecyl radical, and octadecyl radicals, such as the n-octadecyl radical; aryl radicals, such as the phenyl, naphthyl and anthryl radical; haloalkyl radicals, such as the chloromethyl, trichloromethyl, trifluoromethyl, pentafluoroethyl and heptafluoropropyl radical; haloaryl radicals, such as the bromophenyl, chlorophenyl, fluorophenyl, difluorophenyl, fluoro(trifluoromethyl)phenyl, pentafluorophenyl, (trifluoromethyl)phenyl, (trifluoromethylthio)phenyl and trifluoromethoxyphenyl radical.
The abovementioned examples of the radical R4 also apply in their entirety to the radicals R5 and R6.
Examples of hydrocarbons R4, R5 and R6 interrupted by at least one oxygen atom and/or sulfur atom are xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH3, xe2x80x94CH2xe2x80x94CH2Oxe2x80x94CH2CH3 and xe2x80x94CH2xe2x80x94CH2xe2x80x94Sxe2x80x94CH2CH3.
Examples of the radical R4 interrupted by at least one urethane group and/or ester group, and being able to contain, if desired, hydrophobicizing substituents and/or at least one isocyanate group, are 
Preferred examples of radicals D are 
Preferred examples of radicals E are the methoxy, ethoxy and n-butoxy radical.
Preferred examples of radicals F are methyl, ethyl, propyl, 2-methylpropyl and the n-butyl radical.
Preferred examples of radicals R1 are 
Preferred examples of radicals R2 are the phenyl, 4-methylphenyl, 3-methoxyphenyl and 4-methoxyphenyl radical.
Preferred examples of anions Xxe2x88x92 of a complex metal salt and/or of a strong acid are tosylate, SbF6xe2x88x92, PF6xe2x88x92, BF4xe2x88x92, F3CSO3xe2x88x92, F3CCO2xe2x88x92, AsF6xe2x88x92, ClO4xe2x88x92, HSO4xe2x88x92. For the purposes of the present invention, strong acids embrace, in particular, strong Brxc3x6nsted acids.
Preferred iodonium salts of reduced crystallization tendency are those of the general formula (V) 
where D and Xxe2x88x92 are as defined previously.
Particularly preferred iodonium salts of reduced crystallization tendency are those of the formula (VI) 
and of the formula (VII) 
The crystallization tendency of the iodonium salts modified in this way is substantially reduced in relation to the abovementioned prior art. For example, at room temperature the iodonium salt (VII) is a viscous liquid, whereas the comparable hydroxyl-containing iodonium salt having the formula (VIII) below 
is a powder having a melting point of 91xc2x0 C., which is available under the trade name CD-1012 from Sartomer.
Likewise, the solubility of the iodonium salts of the invention in nonpolar media, such as n-alkanes or siloxanes, is substantially greater than that of the comparable hydroxyl-containing iodonium salts.
For example, the iodonium salt of the general formula (VII) has unrestricted solubility in toluol. In contrast, the comparable hydroxyl-containing iodonium salt of the general formula (VIII) is insoluble in toluol and shows no miscibility with organopolysiloxanes containing epoxy groups.
The invention additionally provides for a process for preparing the iodonium salts having reduced crystallization tendency. Iodonium salts of the invention are readily prepared by reaction with isocyanates. 
where xe2x80x9cPhotoxe2x80x9d denotes a photoinitiator to be modified.
The process of the invention comprises reacting a hydroxyl-containing iodonium salt having the general formula (IX)
[R1xe2x80x94Ixe2x80x94R2]+Xxe2x88x92xe2x80x83xe2x80x83(IX)
in which
I is iodine,
Xxe2x88x92 is an anion of a complex metal salt and/or of a strong acid,
R1 is a radical 
xe2x80x83which
Ar is an aromatic hydrocarbon radical, preferably, having 6 to 14 carbon atoms per radical or is an aromatic hydrocarbon radical containing at least one oxygen and/or sulfur atom and having 5 to 15 ring atoms per radical,
a is 1, 2 or 3,
b is 0, 1 or 2,
c is 0, 1 or 2,
Dxe2x80x2, E and F are each substituents of Ar,
Dxe2x80x2 being a radical of the formula
xe2x80x94(O)xxe2x80x94(R3)yxe2x80x94OH
xe2x80x83where
x is 0 or 1,
y is 0 or 1,
R3 is a linear or branched divalent hydrocarbon radical, preferably, having 1 to 40 carbon atoms per radical, which can be interrupted if desired by at least one oxygen atom and/or one sulfur atom and/or one carboxyl group,
E is a radical of the formula
xe2x80x94Oxe2x80x94R5,
F is a radical of the formula
xe2x80x94R6,
R2 is a radical of the formula 
xe2x80x83where
R5 is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted if desired by at least one oxygen atom,
R6 is a monovalent hydrocarbon radical, preferably having 1 to 18 carbon atoms per radical, which can be interrupted if desired by at least one oxygen atom,
d is 0, 1 or 2, and
e is 0, 1 or 2,
with a reagent containing isocyanate groups.
Preferred hydroxyl-containing iodonium salts are those of the general formula (X) 
where Dxe2x80x2 and Xxe2x88x92 are as defined previously.
Particularly preferred starting materials are iodonium salts of the formula (XI) below 
Examples of reagents containing isocyanate groups, which may be reacted with hydroxyl-bearing iodonium salts, are those of the general formula (XII)
OCNxe2x80x94R4xe2x80x83xe2x80x83(XII)
where
R4 is a monovalent linear, branched and/or cyclic alkyl, aryl, haloalkyl and/or haloaryl radical having 1 to 40 carbon atoms, which can, if desired, be interrupted by at least one oxygen atom and/or at least one sulfur atom and/or at least one urethane group and/or ester group, and which can, if desired, contain hydrophobicizing substituents and/or at least one isocyanate group.
Particularly preferred reagents containing isocyanate groups which may be reacted with hydroxyl-bearing iodonium salts are the following:
alkyl isocyanates, such as methyl, ethyl, propyl, butyl, sec-butyl, cyclohexyl, hexyl, octyl, tert-octyl, decyl, dodecyl and/or octadecyl isocyanate;
aryl isocyanates, such as phenyl, tolyl, dimethylphenyl, phenylethyl, propylphenyl, methoxyphenyl, (heptyloxy)phenyl, phenoxyphenyl, acetylphenyl, nitrophenyl, benzyl, methylbenzyl, methoxybenzyl, dimethyl-m-isopropenylbenzyl, naphthyl, (trifluoromethyl)phenyl, di(trifluoromethyl)phenyl, (trifluoromethylthio)phenyl and/or (trifluoromethoxy)phenyl isocyanate;
haloalkyl isocyanates, such as 2-bromomethyl isocyanate; haloaryl isocyanates, such as chlorobenzyl, chlorophenyl, trichlorophenyl, 4-bromo-2,6-dimethylphenyl, fluorophenyl, difluorophenyl, fluoro(trifluoromethyl)phenyl and/or fluorobenzyl isocyanate; diisocyanates, such as cyclohexane, dicyclohexylmethane 4,4xe2x80x2-, diphenylmethane 4,4xe2x80x2-, hexamethylene 1,6-, isophorone, and/or tolylene diisocyanate;
polymeric isocyanates, such as Desmodur N 3300 from Bayer AG, Desmodur L from Bayer AG, Desmodur E41 from Bayer AG, Desmodur Z 4370 from Bayer AG;
polydimethylsiloxane isocyanates, such as 
The iodonium salts of the invention are suitable, for example, as photoinitiators for the polymerization of cationically polymerizable organic substances, such as epoxides, vinyl ethers, organopolysiloxanes containing epoxy groups, organopolysiloxanes containing alkenyloxy groups, such as vinyloxy groups or propenyloxy groups, and olefins. Such substances are described, for example, in U.S. Pat. No. 5,057,549, DE-A-40 02 922, and in the patent documents cited at the outset.